The field of the invention is that of rolling oil filtration in cold-rolling mills, and more specifically “Sendzimir” rolling mills, in particular twenty-cylinder rolling mills, known under the name “20-Hi” for a person skilled in the art; an example of the embodiment of such a twenty-cylinder rolling mill is illustrated in the documents U.S. Pat. Nos. 5,193,377 and 5,471,859. In such a rolling mill, the cylinders are distributed into one lower group and one upper group; more specifically, these groups present a symmetrical structure and each comprise ten cylinders of which: one working cylinder, two first intermediate cylinders, three second intermediate cylinders, and four support cylinders, or support rollers, which are outside of the arrangement.
In a rolling mill, in particular in a 20-cylinder rolling mill, rolling operations are carried out by projecting the oil over the cylinders and the strip. This lubrication and/or cooling operation is implemented by a recycling facility which comprises, successively, positioned between a rolling mill dirty oil outlet and an inlet for clean oil, a circuit comprising:                a dirty oil tank,        a cartridge filtration system,        a clean oil tank.        
A first pumping system, upstream the filtration system following the direction of the oil flow, is joined to the filtration system and enables oil to flow to the filtration system.
A second pumping system, located downstream the filtration system, enables the oil to be sucked through the filtering medium and the push the recirculated oil to the clean oil tank.
A third pumping system, located downstream the clean oil tank, enables oil to flow to the rolling mill. Such general architecture of the recycling and filtration facility is known from document CN 203355436U, and is implemented industrially for 20-cylinder rolling mills from the 1970s. Traditionally, the facility can again comprise exchangers intended to control the temperature of recycled oil as well as control valves enabling the fluid flow to be controlled before it enters the rolling mill.
Since this date, it is known to use, for the filtration system, several plate filtration devices, often known by a person skilled in the art under the name “Supamic Filter”, from the name of its original manufacturer, and positioned on the circuit in a way to achieve oil filtration at the same time. Such a plate filtration device is illustrated in a diagram in FIG. 1 of the document CN 203355436U, identified by marker 5.
The document CN 203154922U describes in more detail the structure of a plate filtration device typically used for filtering Sendzimir rolling mills from the 1970s.
Such a filtration device comprises a tank, typically cylindrical inside which several removable “plates” are superimposed, spread out over the height of the tank, and stacked on top of each other, using base systems. Each one of the plate elements is a hollow element, whereon a large number of filtration cartridges are fixed, parallel, typically by screwing from the upper edge of the cartridge onto the corresponding threaded parts of the plate.
The tank comprises an inlet opening for dirty oil, the filtration being achieved through the filtering medium of the cartridge, from outside to inside, the oil filtered by the cartridge being collected in the plate, which is a hollow element, then led to a plate outlet for clean oil. The oil outlets of different plates are respectively connected, watertight, at openings spread out over the height of the tank, using flange sets.
At the end of a certain time of functioning, it is necessary to replace the cartridges of the filtration device; according to the current procedure, the operator starts by opening the lid, on the upper edge of the tank, then removes the different plates, one by one, using a lifting system, of the highest plate, and to the last plate at the bottom of the tank. Once the plate is removed outside of the tank, the different cartridges can be unscrewed and replaced. For each plate, and prior to the operation of lifting the plate, the operator must necessarily go down into the tank to unscrew the screws of the flange sets and remove the seals between the oil outlet of the plate and the corresponding opening of the tank.
In the cold-rolling mill facilities, in order to obtain the desired filtration flow, typically between 4,000 l/min and 27,000 l/min according to the size of the rolling mill, it is known to use several filtration devices at the same time, each one with four or five plates, the number of cartridges per plate typically being 800. Such a size is, to the inventor's knowledge, still used since the 1970s in these rolling oil recycling facilities and with the aim of reducing the total number of filtration devices per recycling facility.
Thus, and still today, the applicant manufactures such five-plate filtration devices, with a diameter of 2.4 meters and a height of 6.1 meters. Each plate holds 832 cartridges, each one with a length of 13 inches (that is a length of 33.02 cm). The useful volume of the tank is around 18 m3.
The filtration cartridges used often comprise a nylon or fibreglass-based filtering medium. In order to extend the lifespan of the cartridges, back flushable-type filter cartridges are used.
The unclogging of cartridges is carried out at regular time intervals, and is implemented, on plate filtration devices, one by one. For this purpose, a fluid back flush is implemented, typically with compressed air, on one of the plate filtration devices, and while the other plate filtration devices, positioned parallel, ensure the filtration of the rolling oil. For each device, the unclogging is implemented plate by plate, and not simultaneously on all the cartridges of different plates. According to the inventor's observations, the higher the number of plates, the more time needed for the cycle to unclog the cartridges. For example the time necessary to unclog the cartridges of one single filtration device is around 30 minutes when there are five plates.
During the implementation of this unclogging, the filtration capacity of the facility is cut down by that of one device, the overall filtration flow of the facility decreasing. The dirty oil and clean oil reservoirs thus constitute buffer reservoirs, enabling a constant nominal oil flow to the rolling mill: in concrete terms and during the implementation of the back flush, the oil level in the dirty oil reservoir increases, and the level in the clean oil reservoir decreases, these two reservoirs being sized, of sufficient volume, in a way to allow these variations in volume.
More specifically, the volume of the clean oil reservoir is directly related to the time parameters and flow variation, due to the unclogging of a filtration device.
Thus, if:                the parameter T represents the time needed to unclog the cartridges of a filtration device and,        the parameter ΔD, the fall in filtration flow,        
then the clean oil reservoir must be of sufficient volume in a way to maintain a constant nominal flow to the rolling mill, by comprising the fall in filtration flow for the whole unclogging time T: the volume of this reservoir is therefore at least equal to a volume equal to T×ΔD (plus a safety margin) and in order to ensure a constant oil flow to the rolling mill. The dirty oil reservoir must be able to hold such a volume of fluid. Further still, the dirty oil reservoir must still be used, when maintaining on all the filtration devices, as a container when the tanks of different devices are fully emptied: in such a case where the volume of the dirty oil reservoir must be at least equal to the sum of the useful volumes of the tanks of different filtration devices.
Thus, according to the inventor's observations, state of the art recycling facilities are a very significant volume, and in particular because of the size of the clean oil and dirty oil buffer reservoirs. This volume obligates recycling facility equipment to be concentrated at a level lower to the rolling mill, in the cellar, and not on the same level as the rolling mill.
Thus, in state of the art recycling facilities, only the filtration system (that is, plate filtration devices) is installed on the same level as that of the rolling mill, and is easily accessible, the other equipment, namely the dirty oil reservoir, the clean oil reservoir, as well as the different pumps, exchangers and valves thus being installed in a cellar of consequent size and dedicated to the integration of all this equipment.
According to the inventor's observations, this wide cellar represents, during the building of the rolling mill workshop, a significant cost, because of the civil engineering costs, particularly because of the volume of materials (concretes), as well as fire safety arrangements, as well as the size of the cellar's ventilation system. Another disadvantage of such a design is that the equipment present in the cellar is often not very accessible, for maintenance.
According to the inventor's observations, such a recycling facility design, such as it has been known since the 1970s and still respected up to today, can be improved.